Centrifugal pumps for viscous fluid, particularly those containing solids, have been used which include an essentially conical rotor having a helical vane increasing diameter from its entrance end to its discharge end, the vane confronting a wall of the pump housing to form with the pump housing an intake or suction chamber, and a discharge or pressure chamber. The peripheral surface of the vane is fitted as close to the pump housing as possible to minimize backflow between the discharge chamber and the intake chamber.
Shearing forces develop in the gap between the peripheral surface of the vane and the pump housing wall, such shearing forces increase with viscosity and also with decrease in the dimensions of the gap and increase in the area of the peripheral surface of the vane. If the viscous material is contaminated with solid particles, such particles tend to accumulate between the peripheral surface of the vane and the confronting wall surface to the extent that rotation may be blocked.